This invention relates to an optical waveguide photocathode for converting optical signals to electrical signals.
Typically, optical signals are converted to electrical signals in a semiconductor photodiode or a vacuum photodiode. The semiconductor photodiode consists of a sandwich of planar layers of semiconductor material, across which layers a voltage differential is applied. Typically, the first layer is transparent to light to allow light to penetrate into the semiconductor layers where it is absorbed, thereby releasing current carriers which, as a result of the voltage differential, migrate through the layers and complete the circuit between the layers.
The vacuum photodiode consists of a semiconductor between two conductors. In a vacuum photodiode the first conductor is transparent to allow light to pass through to the semiconductor where it can be absorbed, thereby emitting electrons. The second conductor is separated from the semiconductor by a gap and the semiconductor and conductors are placed in a vacuum enclosure. A voltage differential is applied between the two conductors so that the electrons emitted from the semiconductor cross the gap to the second conductor. The semiconductor in a vacuum photodiode is commonly referred to as the photocathode.
The disadvantage of the semiconductor photodiode is that the carriers of current, for example electrons, are bound to the semiconductor and cannot be emitted into a vacuum for use in applications such as streak tubes and photomultipliers. A streak tube is a device for performing high speed analysis of signals. A photomultiplier is a device which increases the gain of the signal.
While the vacuum photodiode permits the emission of electrons into a vacuum for use with a streak tube or photomultiplier, the disadvantage of a typical vacuum photodiode is that it is relatively insensitive and inefficient. The insensitivity comes from the fact that these devices typically have a relatively large dark current. The dark current consists of those electrons released by the semiconductor in the absence of any optical signal source. The dark current is produced by thermal and quantum effects and is always present. For a given semiconductor material the amount of dark current is directly proportional to the size of the active area of the semiconductor. The active area is that part of the semiconductor upon which the optical signal shines and from which released electrons are collected by the conductors.
The relatively large dark current of the typical photocathode methods for converting optical signals to electrical signals masks the minute currents generated by very low level optical signals, thereby rendering such devices insensitive to such low level signals.
In addition to being insensitive to low level signals because of a relatively large dark current, the typical photocathode methods are relatively inefficient in converting optical signals into electrical signals because they must use only a very limited range of semiconductor materials. These materials are able to efficiently convert optical signals into electrical signals only within a very limited range of wavelengths of light. In the typical photocathode methods for converting optical signals to electrical signals, only those semiconductors having a relatively low work function can be used. The work function is the energy required to free an electron bound to an atom of the semiconductor material and move it an infinite distance from that atom. As a practical matter this is the energy needed for the electron to migrate into the second conductor. A semiconductor with a low work function is required in present devices because the electrical fields which move the electrons through the semiconductor to the second conductor are relatively weak and such fields are not strong enough to move the electron through a semiconductor with a high work function.
The consequence of using such a limited range of semiconductor materials is that the semiconductor material cannot be selected for maximum efficiency in converting light into electrical signals. For example, the semiconductor typically used in photocathode devices is either silver cesium oxide or antimony cesium oxide, and the efficiency of these materials is such that at a wavelength of 1,300 nanometers only approximately one electron is released for every one million photons of incident light. Thus, 999,999 photons out of every 1 million photons of the optical signal are not detected and do not get converted into an electrical signal. Also, because of the limited number of semiconductor materials which will function in the low electric fields of such typical devices the semiconductor cannot be chosen to optimize its efficiency for a given wavelength of light.
Accordingly, there is a need for a more sensitive and efficient method of converting optical signals to electrical signals, particularly in streak tubes and photomultipliers.